Distance-controlled tunneling transducer and direct access storage unit employing the transducer

ABSTRACT

A distance-controlled tunneling transducer comprises a plurality of tunnel tips arranged in an array at a tunneling distance from an electrically conductive surface of a storage medium. Each tip is attached to a respective cantilever beam permitting the distance between each tip and the surface to be individually pre-adjusted electrostatically. Arranged in juxtaposition with each cantilever beam is an active control circuit for adjusting the tip-to-surface distance during operation of the storage unit, thus preventing crashes of the associated tip into possible asperities on the surface of the recording medium. Each control circuit is designed such that its operating voltage concurrently serves to pre-adjust its associated cantilever beam and to maintain the tip-to-surface distance essentially constant.

This application is a division of application Ser. No. 07/421,207, filedOct. 13, 1989, now U.S. Pat. No. 5,043,577.

BACKGROUND OF THE INVENTION

The present invention relates to a distance-controlled tunnelingtransducer for use in a direct access storage unit, having a pluralityof tunnel tips arranged facing a recording medium. In particular, theinvention teaches improved gap control means for implementation inmicromechanical techniques. The invention is also applicable tolow-voltage field-emission environments where the gap dimension issomewhat larger than in the tunneling regime. Therefore, where in thefollowing description reference is made to tunneling phenomena, thoseskilled in the art will be able to easily apply what is said tofield-emission phenomena as well.

In the tunneling regime, the tip/surface distance typically is less than2 nm, and in the field-emission environment, that distance is consideredto be on the order of 20 nm. Small local deviations from planarity ofthe surface of the recording medium, say on the order of tenths of ananometer, may result in relatively large changes of the tip current, inparticular in the tunnel regime, where the dependence of the tunnelingcurrent on the tip-to-surface distance is exponential. Because of thefact that the operating distances in tunneling as well as infield-emission environments are so small, it may even happen that thetip crashes into a surface asperity and thus suffers damage, unless somemeasure is taken to maintain the tip-to-surface distance essentiallyconstant. In conventional tunneling microscopes, this problem is solvedby means of a feedback loop operating from the tip distance above thesurface, with the aim of keeping the tip current constant. It will beobvious to those skilled in the art that in view of the relatively largevelocity with which surface asperities may be encountered as the tipscans across the surface of the recording medium, and because of thepossibly abrupt change in tip-to-surface distance, a very fast responseof the feedback loop is required.

Recently, direct access storage devices have been proposed which operateon the principle of the scanning tunneling microscope. The basicreference in this area is EP A-0 247 219. The reference teaches astorage unit comprising an array of tunnel tips arranged at a tunneldistance from the surface of a recording medium which is capable ofpermitting digital information to be written or read through variationsof the tunneling current. The tunnel distance is maintained by means ofa feedback loop, and the idea of integrating the control circuitry ofthat feedback loop on the tunnel tip array is mentioned. No details ofthe circuitry nor of the way the integration can be achieved are,however, given.

A scanning tunneling microscope realized in micromechanical techniquesis disclosed in U.S. Pat. No. 4,520,570. A semiconductor chip has slotsetched into it in a pattern resulting in a plurality of tunnel tips tobe formed that are hinged by stripes of semiconductor material to themain body of the chip. Again in this reference, an area is provided onthe semiconductor chip to contain control circuitry associated with thetunnel tip, in casu a transistor acting as an impedance transformer.

While it is acknowledged that the idea has occurred to integrate thetunnel tip feedback loop into the semiconductor chip on which the tunneltip is formed, it has turned out that conventional control circuitry forscanning tunneling microscope and field-emission microscope applicationsis by far too complex and, hence, too bulky to be installed on thesemiconductor chip if the chip is to carry a plurality of tips arrangedin a small array.

As a possible alternative, one may require that both the array of tipsand the surface of the recording medium be sufficiently flat, forinstance within 0.1 nm over the area to be scanned, to allow for globalgap width control averaging over the currents of all tips. Such arequirement would, however, impose undesirable constraints with regardto precision of manufacturing and alignment, as well as to the choice ofmaterials for the recording medium.

SUMMARY OF THE INVENTION

A primary object of the present invention is to solve this problem byforegoing the idea of perfect feedback regulation, i.e. with zero error.Instead, an open loop circuit is provided which compensates for distancevariations though less completely, but which is sufficiently simple tobe easily integrated into a multiple-tip scanner head. The achievablereduction in distance variation as result of variation of thetip-to-surface distance typically is a factor of from 30 to greater than100. The result is relaxed manufacturing tolerance requirements withregard to flatness of the recording medium to values that can easily bemaintained.

Details of an embodiment of the invention will hereafter be described byway of example having reference to the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a top plan view onto a section of an array of tunnel tips;

FIG. 2 is an enlarged cross-section of the tunnel region along line A--Aof FIG. 1;

FIG. 3 is a schematic representation of the tunnel tip, showing thedifferent positions of the cantilever beam during operation;

FIG. 4 is a circuit diagram of the circuitry associated with the tunneltip;

FIG. 5 is a graph used in the determination of the operating point ofthe circuitry of FIG. 4;

FIG. 6 is a section of the graph of FIG. 5 having a larger scale;

FIG. 7 shows a characteristic of the control circuitry in accordancewith the parameters chosen; and

FIG. 8 shows another characteristic of the control circuity inaccordance with the parameters chosen.

DETAILED DESCRIPTION

While the invention will be described in connection with theelectrostatic deflection of a cantilever beam as conventionally used inmicromechanical arrays, it will be obvious to those skilled in the artthat the invention is also applicable to piezoceramic scanners.

FIGS. 1 and 2 show the contemplated arrangement of the elements of thetransducer in accordance with the invention in a semi-schematical way.Referring to FIG. 1, there is shown a section of the transducer 1 withthree tunnel tips 2', 2", 2'" out of a plurality of tunnel tips (orfield-emission tips) arranged in an array. The tunnel tips are attachedto cantilever beams 3', 3", 3'" respectively which are formed, e.g. byetching, from body 4, FIG. 2, of transducer 1 as an integral partthereof. Transducer 1 is mounted to a conventional xyz-drive 5 whichprovides lateral deflection as well as coarse approach and adjustment ofthe average distance between tunnel tip 2 and the oppositely disposedsurface 6, by keeping the total tunneling current essentially constant.Surface 6 may actually be the surface of a sample to be inspected bymeans of a scanning tunneling microscope. However, since the presentinvention is intended particularly for use in connection withinformation storage devices, for the purposes of this descriptionsurface 6 will be assumed to be the surface of a recording medium 7. Themedium 7 may comprise, for example, a magnetizable material. Since thefunnel effect requires the surface opposite by the tunnel tip to beelectrically conductive, any non-conductive material used as therecording medium may be provided with a very thin conductive coating.

Each of the cantilever beams can be deflected electrostatically byapplication of voltages U₁ and U₂, respectively, between an electrode8', 8", 8'" on cantilever beam 3', 3", 3'" and a counter-electrode 9 atthe bottom of the recess 10 etched into body 4 underneath eachcantilever beam, and between the electrode 8 and the surface 6 of therecording medium 7. This deflection is used to control the width of thetip-to-surface gap, in particular during operation in the tunnelingmode.

Arranged on body 4 of transducer 1 are electric circuit elements 11through 17 which serve to control the deflection of the cantilever beams3 in the array and to create the tunneling currents across the gapsbetween the tunnel tips and the surface 6 of the common recording medium7. FIG. 3 is an enlargement of a portion of FIG. 2 to better show therelevant z-coordinates, distances and voltages. In its home position,i.e, in a voltageless state, cantilever beam 3 assumes a positiondesignated in FIG. 3 as z_(CB) at a distance Z₁ from counter-electrode 9the surface of the counter-electrode being located at Z_(BOT), and at adistance Z₂ from the surface 6 of recording medium 7 which is located atZ_(S). In the home position of tunnel tip 2, and considering that thetip has a height dimension Z_(P), its apex is located at Z_(TIP), i.e. agap width Z_(G) away from surface 6.

Application of voltages U₁, U₂ between electrode 8 on cantilever beam 3and surface 6 causes the cantilever beam 3 to deflect by a distance ΔZto a new position Z_(CB0). The voltages required for deflectingcantilever beam 3 are provided by circuitry comprising a field-effecttransistor 11 connected between a supply line 12 and electrode 8 oncantilever beam 3. Field-effect transistor 11 acts as a constant currentsource which is set by means of a gate line 13. Counter-electrode 9 ispart of supply line 12 and is at a constant potential U₀. Hence thedeflection voltages are U₁ =U₀ -U_(t), U₂ =U_(t).

When tunnel tip 2 is far from surface 6, i.e. gap Z_(G) is large, thenthe resistance across the tunnel gap R_(t) →∞, U₁ →0, U₂ →U₀. Ascantilever beam 3 becomes most deflected towards surface 6 of recordingmedium 7: ΔZ_(max) ≡B(U₀ /Z_(P))², where it is assumed that Z_(G)<Z_(P). The term B will be explained below.

When tip 2 gets closer to surface 6 so that tunneling current I_(t)becomes finite, i.e. larger than the unavoidable leakage current offield-effect transistor 11, which is typically <100 pA, then the voltageratio U₁ /U₂ increases resulting in a retraction of cantilever beam 3.Thus, any increment in z_(s), that is, for example, some surfaceroughness, produces a much smaller decrease of Z_(G).

FIG. 4 is a circuit diagram a the unit cell, viz. for the electroniccircuitry associated with one cantilever beam 3. The tunneling currentI_(t) flowing across the tip-to-surface gap, i.e. through the tunnelresistance R_(t), is monitored by means of a load resistance 14 (R_(L))which is chosen such that R_(L) <R_(t) under operating conditions. Thesignal U_(SIG) occurring across load resistance 14 is provided via lines15 and 16. A load capacitance 17 (C_(L)) introduces some inertia intothe compensation process characterized by a time constant τ_(L) =R_(t)C_(L). Hence, information-carrying variations passing tip 2 within atime shorter than the time constant τ_(L) are not compensated, givingrise to a large variation in U_(SIG). The performance of the scheme isdescribed by the Z_(G) (Z_(S)) characteristics under quasi-staticconditions (with reference to τ_(L)). Therefore, R_(L) and C_(L) areignored in this part of the discussion. Further ignored are the straycapacitance C₁ between cantilever beam 3 and supply line 12, and thestray capacitance C₂ between cantilever beam 3 and counter electrode 9,because they are negligible compared to load capacitance C_(L). Withoutloss of generality, it can be assumed that Z_(CB0) ≡0, hence, Z_(S) <0.Under these conditions, Z_(G) (Z_(S)) can be derived from the followingset of relations:

    Z.sub.G =-Z.sub.P -Z.sub.S +ΔZ                       [1]

    ΔZ≃B[(U.sub.1 /Z.sub.1).sup.2 -(U.sub.2 /Z.sub.2).sup.2 ]                                                         [2]

    Z.sub.1 =Z.sub.BOT -ΔZ                               [3]

    Z.sub.2 =-Z.sub.S +ΔZ                                [4]

    U.sub.1 =U.sub.0 -U.sub.2                                  [ 5]

    U.sub.2 =U.sub.t =I.sub.t R.sub.t                          [ 6]

    R.sub.t =R.sub.t0 e.sup.κZ.sbsp.G                    [ 7]

    B=εε.sub.0 I.sup.4 /4Et.sup.3,             [8]

with εε₀ =dielectric constant (≃0,8 pF/m), E=elastic modulus, l=lengthand t=thickness of the cantilever beam 3, respectively, and R_(t0) ≃40kω, κ≃10¹⁰ m⁻¹.

Equations [1] through [7] cannot be solved analytically for Z_(G) butthe derivative can be determined easily: ##EQU1##

With the parameters chosen below, and at realistic operating conditionsin the tunneling mode, Z_(G) <ΔZ<Z₁,2 such that Z₁ ≃z_(BOT) and Z₂≃Z_(p) ; further U₁ ≃U₀ <U₂ =U_(t), the quantity A becomes:

    A≃-2κBU.sub.0 U.sub.t /Z.sub.BOT.sup.2 [ 11]

For a numerical calculation, the following values have provenappropriate: ε=1; ε₀ =8×10⁻¹² F/m; l=200 μm; t=2 μm; E=10¹¹ N/m²(silicon, quartz); z_(BOT) =Z_(P) =3 μm. With these values and aprojected width w=200 μm of the cantilever beam, a spring constant ofC*=4,5 N/m results which is sufficient to prevent mechanicalinstabilities due to interfacial forces. The first elastic resonance ofthe cantilever beam occurs at >100 kHz which is much better than inpresent-day scanning tunneling microscopes. The deflection parameterbecomes:

    B=0,4×10.sup.-20 m.sup.3 /V.sup.2                    [ 12]

Assuming operation at U_(t) =0.5 V, then A≃100. Since A>1, one mayignore the 1 in the denominator of equation [10], hence ##EQU2##

Equation [13] means that a 10 nm variation in surface height results inno more than a 0.1 nm change in tunnel gap width.

For very small and very large tunnel gap widths, the above-given set ofequations is easily solved for z_(S) (Z_(G)). There is no reductioneffect to be expected: ##EQU3##

FIG. 5 depicts the linear and almost linear relations calculated inaccordance with Equation [14]. The dashed lines designated a, b, and c,having a slope of -1, and being mutually parallel-displaced by theamounts ΔZ_(max), and 2 ΔZ_(max), respectively, are the calculatedasymptotes to the curve z_(S) (Z_(G)) represented in a semi-quantitativeway by the solid curve d. The operating point is to be chosen on thehorizontal plateau (of the width 2 ΔZ_(max)) between dashed lines a andb. Since the voltage U₁ is generally kept small, the operating pointwill preferably be chosen near the right-hand end of the plateau.

For this purpose, the tunneling current from a few selected cells can befed into a regular feedback control circuit. This measure assures thatthe overall system settles approximately at the operating points of theindividual elements. The quantitative behavior of the response curve maybe obtained by numerical integration of Equation [13]. The initialsetting of the parameters may be chosen as follows:

    Z.sub.G =O, R.sub.t =R.sub.t0, U.sub.2 =I.sub.t R.sub.t0 <U.sub.1 =U.sub.0, ΔZ=B(U.sub.0 /Z.sub.p).sup.2, z.sub.s =ΔZ.

The resulting characteristics are shown in FIGS. 6 through 8. FIG. 6depicts the relevant section of curve d in FIG. 5, namely the relationof tunnel gap width versus sample position, for three values of thetunneling current, viz. 0.1 nA, 1 nA, and 10 nA, at a larger scale. FIG.7 shows the corresponding relation between dZ_(G) /dz_(S) versus U_(t)which is independent of I_(t). It can be seen that dZ_(G) /dz_(S) ≃0.01in the operating range around U_(t) =0.5 V. FIG. 8 relates the gap widthto the tunneling voltage for the three values 0.1 nA, 1 nA and 10 nA ofthe tunneling current I_(t).

It will be apparent from the foregoing description that variations intunneling current occurring fast as compared to the time constant τ_(L)of the R_(L) -C_(L) circuit are not compensated for by the currentstabilizer comprising field-effect transistor 11. Small asperities onthe surface 6 of the recording medium 7 (topography) as well as localchanges in workfunction of the recording material, therefore, will showup in full strength in the tunneling current I_(t). They create avoltage signal U_(SIG) across load resistance R_(L) 14 which can be usedfor further processing of the stored information.

While the present invention is not directed to the storage medium perse, it seems in order to briefly introduce a storage medium capable ofrecording variations of the tunneling current. A suitable storage mediummay, for example, comprise a thin layer of silicon dioxide on top of asilicon substrate. The oxide is prepared to have a plurality of trappingsites of about 2.4 eV depth. Electrons emitted from a tunnel tip can bestably trapped at such sites at room temperature (cf. D. J. Dimaria inPantelides (Ed.) "The Physics of SiO2 and its Interfaces", Pergamon1978, p. 160 et seq.). This mechanism qualifies for write/readapplications.

On conventional storage disks, the storage locations are arranged inconcentric circles or tracks about a single common center. By contrast,on the storage medium proposed for use in connection with the presentinvention, the storage locations are arranged in a plurality ofidentical groups, with all storage positions in any one group beingarranged in concentric circles or tracks about the group's own center,and with all centers being arranged in an ordered array on the recordingsurface of the storage medium. The concentric circles or tracks ofstorage locations of each group form a "microdisk" with a diameter ofless than 0.1 mm. Even with several hundred "microdisks" per recordingsurface, the area required for a given storage capacity is much smallerthan required in any other known storage devices.

The storage medium just described may be attached to the free end of anelongated piezoceramic bendable tube, such as the one known from EP-A-0247 219, the other end of which is rigidly anchored. Closely adjacentthe recording surface at the free end of the tube is mounted thetransducer of the present invention with each tunnel tip of its array oftunnel tips being aligned with one of the microdisks. Each tunnel tipfaces, and is spaced closely adjacent to, the recording surface on thestorage medium. The distance (in the nanometer range) between eachtunnel tip and the recording surface is at the beginning individuallyadjusted so that each tip normally is disposed at the same preselectedtunneling distance from the recording surface 6.

By successively energizing electrode pairs attached to the tube, thefree end of the tube and, hence, the recording surface is forced to movein a circular orbit. The diameter of this orbit will vary according tothe potential differential that is selected. Thus, the recording surfaceat the free end of the piezoceramic tube can be caused to move in anyone of a plurality of concentric orbits relative to the tunnel tips ofthe transducer array. As a result, each tip will scan that one of theplurality of concentric circles of tracks of its microdisk correspondingto the selected orbit diameter of the tube. Digital information iswritten into and/or read out of the storage medium by varying thetunneling current flowing across the gap between the tunnel tip andrecording surface.

Recording, therefore, involves addressing and moving a selectable one ofthe tips 2 in the array of transducer 1 and concurrently energizing theelectrode pairs surrounding the tube to a potential corresponding to anorbit diameter that permits access to a desired one of the concentrictracks on the associated microdisk.

It will be clear to those skilled in the art that several otherrecording media, as well as other known schemes for the mutualdisplacement of transducer and/or tunnel tips and recording medium maybe employed to achieve the desired result. The important point is thatwith all those schemes the control circuitry necessary to compensate thevariations in the distance between each individual the tunnel tip andthe surface of the recording medium is integrated on the transducer.

What is claimed is:
 1. A direct access storage unit comprising arecording medium; a plurality of tunnel tips arranged on a transducer inproximity to said recording medium, and means for generating a mutualperiodic displacement between said recording medium and said transducer,wherein each tip is operated as a detector for the current flowingacross a gap (Z_(G)) between said tip and a surface of said recordingmedium, and wherein each tip is attached to a respective cantilever beamwith the mutual distance between said tip and the surface of therecording medium being adjustable by electrostatic means, characterizedin that each tip is associated with an active control circuit integratedon said transducer in proximity to the respective cantilever beamassociated with a particular tip and comprising a transistor, a loadresistance, and a capacitance, and that the circuit is connected suchthat the supply voltage (U₀) creating the current across the gap betweensaid tip and the surface of said recording medium also serves to adjust,during operation, the gap distance (Z_(G)) between said tip and thesurface of said recording medium.